The vegan diet restricts all animal products for ethical, environmental, or health reasons.

Veganism is also associated with resistance to animal exploitation and cruelty.

How it works: Veganism is the strictest form of vegetarianism.

In addition to eliminating meat, it eliminates dairy, eggs, and animal-derived products, such as gelatin, honey, albumin, whey, casein, and some forms of vitamin D3.

Weight loss: A vegan diet seems to be very effective at helping people lose weight — often without counting calories — because its very low fat and high fiber content may make you feel fuller for longer.

One 18-week study showed that people on a vegan diet lost 9.3 pounds (4.2 kg) more than those on a control diet. The vegan group was allowed to eat until fullness, but the control group had to restrict calories (13).

However, calorie for calorie, vegan diets are not more effective for weight loss than other diets (14).

They seem to be very effective at reducing dangerous belly fat, which can become lodged around your organs (35, 36).

People on very low-carb diets commonly reach a state called ketosis. Many studies note that ketogenic diets lead to more than twice the weight loss than a low-fat, calorie-restricted diet (35, 37, 38, 39).

Other benefits: Low-carb diets tend to reduce your appetite and make you feel less hungry, leading to an automatic reduction in calorie intake (40, 41).

In extremely rare cases, very low-carb diets can cause a serious condition called ketoacidosis. This condition seems to be more common in lactating women and can be fatal if left untreated (47, 48, 49, 50).

However, low-carb diets are safe for the majority of people.

SUMMARY Low-carb diets severely limit carb intake and push your body to use fat for fuel. They aid weight loss and are linked to many other health benefits.

Other benefits: Apart from weight loss, there are no recorded benefits of the Dukan diet in scientific literature.

The downside: There is very little quality research available on the Dukan diet.

The Dukan diet limits both fat and carbs — a strategy not based on science. On the contrary, consuming fat as part of a high-protein diet seems to increase metabolic rate compared to both low-carb and low-fat diets (60).

Studies reveal that this diet is ineffective for weight loss in the long term.

Proponents of the ultra-low-fat diet claim that traditional low-fat diets are not low enough in fat and that fat intake needs to stay under 10% of total calories to produce health benefits and weight loss.

How it works: An ultra-low-fat diet contains 10% or fewer calories from fat. The diet is mostly plant-based and has a limited intake of animal products (66).

Therefore, it's generally very high in carbs — around 80% of calories — and low in protein — at 10% of calories.

Weight loss: This diet has proven very successful for weight loss among obese individuals. In one study, obese individuals lost an average of 140 pounds (63 kg) on an ultra-low-fat diet (67).

Another 8-week study with a diet containing 7–14% fat showed an average weight loss of 14.8 pounds (6.7 kg) (68).

Other benefits: Studies suggest that ultra-low-fat diets can improve several risk factors for heart disease, including high blood pressure, high cholesterol, and markers of inflammation (69, 70, 71, 72, 73, 74).

Surprisingly, this high-carb, low-fat diet can also lead to significant improvements in type 2 diabetes (75, 76, 77, 78).

Furthermore, it may slow the progression of multiple sclerosis — an autoimmune disease that affects your brain, spinal cord, and optic nerves in the eyes (79, 80).

The downside: The fat restriction may cause long-term problems, as fat plays many important roles in your body. These include helping build cell membranes and hormones, as well as helping your body absorb fat-soluble vitamins.

Moreover, an ultra-low-fat diet limits intake of many healthy foods, lacks variety, and is extremely hard to stick to.

SUMMARY An ultra-low-fat diet contains less than 10% of its calories from fat. It can cause significant weight loss and may also have impressive benefits for heart disease, type 2 diabetes, and multiple sclerosis.

The HCG diet is an extreme diet meant to cause very fast weight loss of up to 1–2 pounds (0.45–1 kg) per day.

Its proponents claim that it boosts metabolism and fat loss without inducing hunger (82, 83).

HCG (human chorionic gonadotropin) is a hormone present at high levels during early pregnancy.

It tells a woman's body it's pregnant and maintains the production of hormones that are important for fetal development. It has also been used to treat fertility issues (84).

How it works: The diet is split into three phases. During the first phase, you begin taking HCG supplements.

During the second phase, you follow an ultra-low-calorie diet of only 500 calories per day, along with HCG supplement drops, pellets, injections, or sprays. The weight loss phase is prescribed for 3–6 weeks at a time.

In the third phase, you stop taking HCG and slowly increase your food intake.

Weight loss: The HCG diet does cause weight loss, but multiple studies conclude that the weight loss is due to the ultra-low-calorie diet alone — not the HCG hormone (82, 85, 86, 87).

Furthermore, HCG was not found to reduce hunger.

Other benefits: Aside from weight loss, there are no documented benefits of the HCG diet.

The downside: Like most other ultra-low-calorie diets, the HCG diet may cause muscle loss, which results in a reduced ability to burn calories (61).

Such severe calorie restriction further reduces the number of calories your body burns. This is because your body thinks it's starving and therefore attempts to preserve energy (63).

In addition, most HCG products on the market are scams and don't contain any HCG. Only injections are able to raise blood levels of this hormone.

Moreover, the diet has many side effects, including headaches, fatigue, and depression. There is also one report of a woman developing blood clots, most likely caused by the diet (83).

The FDA disapproves of this diet, labeling it dangerous, illegal, and fraudulent (88).

SUMMARY The HCG diet is a rapid weight loss diet. It's not based on any scientific evidence and may reduce metabolic rate and cause muscle loss, headaches, fatigue, and depression.

How it works: The Zone Diet recommends balancing each meal with 1/3 protein, 2/3 colorful fruits and veggies, and a dash of fat — namely monounsaturated oil, such as olive oil, avocado, or almonds.

It also limits high-GI carbs, such as bananas, rice, and potatoes.

Weight loss: Studies on low-GI diets are rather inconsistent. While some say that the diet promotes weight loss and reduces appetite, others show very little weight loss compared to other diets (90, 91, 92, 93).

Other benefits: The greatest benefit of this diet is a reduction in risk factors for heart disease, such as reduced cholesterol and triglycerides (92, 94, 95, 96, 97).

The 16/8 method: Involves skipping breakfast and restricting your daily eating period to eight hours, subsequently fasting for the remaining 16 hours of the day.

The eat-stop-eat method: Involves 24-hour fasts once or twice per week on non-consecutive days.

The 5:2 diet: On two non-consecutive days of the week, you restrict your intake to 500–600 calories. You do not restrict intake on the five remaining days.

The Warrior Diet: Eat small amounts of raw fruits and vegetables during the day and one huge meal at night.

How it works: Intermittent fasting is commonly used for weight loss because it leads to relatively easy calorie restriction.

It can make you eat fewer calories overall — as long as you don't overcompensate by eating much more during the eating periods.

Weight loss: Intermittent fasting is generally very successful for weight loss. It has been shown to cause weight loss of 3–8% over a period of 3–24 weeks, which is a lot compared to most weight loss diets (99, 100).

In addition to causing less muscle loss than standard calorie restriction, it may increase your metabolic rate by 3.6–14% in the short term (99, 101, 102, 103).

In addition, some people should avoid fasting, including those sensitive to drops in blood sugar levels, pregnant women, breastfeeding moms, teenagers, children, and people who are malnourished, underweight, or nutrient deficient.

SUMMARY Intermittent fasting lets your body cycle between fasting and eating. It is very effective for weight loss and has been linked to numerous health benefits.